What you need to know
Symmetry
2D shapes can have what we call lines of symmetry and rotational symmetry.
Line of symmetry – This is a line that you can draw through the shape such that what you see on one side of that line is a “mirror-image” of what you see on the other side.
Rotational symmetry– The order of rotational symmetry a shape has is the number of times the shape can be rotated and still look the same.
You should be happy with the following topics before vising symmetry:
– 2D Shapes and Quadrilaterals
Lines of Symmetry in regular polygons
Below are a selection of common shapes with their lines of symmetry shown as red, dashed lines.
The number of lines of symmetry in a regular polygon is equal to the number of sides.
See below for the number of line of symmetry in common regular polygons
Rotational symmetry
The order of rotational symmetry a shape has is the number of times the shape can be rotated and still look the same.
The pentagon below can be rotated 5 times and look the same.
This means it has rotational symmetry order 5.
If a shape can be rotated 360\degree and only looks the same when it is back at its original position, then it has rotational symmetry order 1.
The rotational symmetry of common shapes is shown below.
Irregular polygons
Many shapes have no lines of symmetry at all, like the triangle shown below.
So, we can conclude that some triangles are symmetrical whilst some are not, this is true of other shapes as well.
The trapezium on the left has 1 line of symmetry, whilst the trapezium on the right has none.
There’s no need to try to memorise all of these shapes and their lines of symmetry. Instead, get used to what a shape looks like when it is symmetrical.
Example Questions
1) State the number of lines of symmetry an equilateral triangle has:
Looking at the picture we can see an equilateral triangle has three lines of symmetry, each one from a vertex to the centre of the opposing side,
2) State the number of lines of symmetry an isosceles triangle has:
Looking at the picture we can see an isosceles triangle only has one line of symmetry, a vertical line from top to bottom.
3) Below is a regular pentagon. Draw all lines of symmetry on the shape
If we recall, a regular pentagon should have 5 lines of symmetry since it has 5 sides. Drawing all 5 lines on, we get the picture below:
4) Draw a shape with exactly 2 lines of symmetry. Include the lines of symmetry on your drawing.
There are a number of possible shapes you could make with two lines of symmetry, the most straightforward being a rectangle.
5) Draw all lines of symmetry on the shape below.
The shape has 8 lines of symmetry .
Worksheets and Exam Questions
(NEW) Symmetry Exam Style Questions - MME
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